Recently, the technologies utilizing supercritical fluids such as a supercritical carbon dioxide, etc. as solvents have been vigorously researched. A supercritical fluid shows a surface tension of zero, and thus can have a diffusivity comparable to gases, and this fluid has a density close to that of liquids and thus can be used as a solvent. As one of the technologies utilizing the physical properties of supercritical fluids, electroless plating of plastics with the use of supercritical fluids is proposed (cf. Non-Patent Publication 1). The electroless plating method with the use of a supercritical fluid is effective to overcome the problems which the conventional electroless plating methods for plastics have confronted.
The electroless plating methods have come into wide use as means for forming electrically conductive metal films on the surfaces of plastic structural bodies for electronic devices. In general, a conventional plastic electroless plating process comprises a resin-molding step, a molded article-degreasing step, an etching step, a neutralizing- and wetting-step, a catalyst-adding step, a catalytic activation step, and an electroless plating step, while this process has some difference depending on the kinds of materials, etc. Roughening of the surface of a plastic molded article in the etching step is effective to improve the adhesion of a plating film formed in the subsequent electroless plating step.
In the etching process, a solution of chromic acid or a solution of alkali metal hydroxide is used as an etchant. A post-treatment such as neutralization of this etchant is needed before the waste thereof. This post-treatment is one of factors to increase the production cost. Again, the etchant for use in the etching step is highly hazardous, and thus, careful attentions should be paid to handling of such an etchant. In the European, RoHS (Restriction of the use of certain Hazardous Substances in electrical and electric equipment) has been constituted. Material manufacturers and component suppliers have been performed the duty to guarantee that no chromium (VI) or the like shall be contained in electric and electronic devices newly put on the European markets, on and after Jul. 1, 2006. Under such a situation, the conventional plastic electroless plating process with the use of an etchant has been required to be shifted to an alternate process therefor.
Non-Patent Publication 1 discloses one method for forming a plating film with sufficient adhesion on a plastic molded article, without the use of any etchant. According to this method of Non-Patent Publication 1, firstly, an organic metal complex is dissolved in a supercritical carbon dioxide, and the supercritical carbon dioxide having the metal complex dissolved therein is brought into contact with a variety of polymer molded articles, so that the organic metal complex is allowed to penetrate the polymer molded articles. Then, the organic metal complex is reduced by a heat treatment or chemical reduction treatment. As a result, fine metal particles precipitate on the polymer molded article, so that electroless plating of the polymer molded article becomes possible by using the fine metal particles as catalytic nuclei. Any treatment of the waste is not needed in the electroless plating process employing the method of Non-Patent Publication 1 because of no use of an echant. In addition, roughening of the surface of the polymer molded article with an echant is not needed, so that the molded article is low in surface roughness and thus is in a good condition.
In the meantime, the present inventors have proposed a new method different from the method of Non-Patent Publication 1, as the electroless plating method without using any etchant (Patent Publication 1). According to the method of Patent Publication 1, fine metal particles of a metal complex or the like are charged in a high-pressure vessel and are dissolved in a supercritical carbon dioxide; this supercritical carbon dioxide is dissolved at the flow-front portion of a thermoplastic cylinder; and a molten resin is injected from this thermoplastic cylinder for molding of the molten resin. By dissolving the metal complex or the like in the resin before the molding, it becomes possible to segregate the fine metal particles as catalytic nuclei for electroless plating, on the surface portion of the molded article (or in the proximity of the surface of the molded article).
The electroless plating process using the supercritical fluid described in the above Non-Patent Publication 1 is a batch process which comprises a step of molding, a step of allowing the organic metal complex to penetrate the molded article, and a step of electroless plating on the molded article in this order. In the organic metal complex penetration step, the molded article and carbon dioxide are needed to be heated and pressurized up to a supercritical state, and after the penetration treatment, the molded article and the carbon dioxide are needed to be again lowered to ordinary temperature and pressure. Therefore, a certain time is required for this heat-pressurizing treatment and the treatment of lowering to the ordinary temperature and pressure. In case where the organic metal complex penetration step of Non-Patent Publication 1 is employed instead of the etching step, it is considered to be very difficult to achieve a high continuous productivity (mass productivity). Particularly in case of production of a large molded article, a high-pressure vessel for use in heating and pressurizing is increased in size, so that the amounts of carbon dioxide and the molded article to be heated and pressurized are increased accordingly. Consequently, decrease in continuous productivity is more remarkable. Besides, there arises another disadvantage when the organic metal complex penetration step of Non-Patent Publication 1 is employed: that is, the supercritical carbon dioxide, etc. tend to soften the surface of the polymer molded article, and the supercritical fluid and the metal complex are allowed to penetrate the softened polymer molded article, with the result that the molded article deforms because of the softening thereof, which makes it hard to maintain and ensure the precision in the external shape and configuration of the molded article.
In contrast, in the molding method of Patent Publication 1, it is possible to carry out the electroless plating process on the molded article subsequent to the resin-molding step. Accordingly, it becomes possible to omit the necessary steps of the conventional process, i.e., the molded article-degreasing step, the etching step, the neutralization- and wetting-step, the catalyst-adding step and the catalyst-activation step. The electroless plating process, therefore, can be greatly simplified. Since the molding method of Patent Publication 1 is free from any problem in continuous productivity (or mass productivity) which Non-Patent Publication 1 suffers from, the electroless plating process for plastics by the molding method of Patent Publication 1 is optimal as an alternate process for commercial production or mass production.